The present invention refers to an apparatus for cutting a plain, curved or arched workpiece by means of a beam-shaped fluid cutting medium ejected under high pressure out of a nozzle. This apparatus comprises a nozzle provided at the end of a tube-like member and means for providing a high-pressure fluid jet escaping from the nozzle.
It is well known in the prior art to cut or separate workpieces in a touchless method by means of a fluid jet, a gas jet or a laser beam, said jet or beam being directed towards the workpiece to be cut and being moved along a desired path. Probably the most common apparatus of this kind are the water jet cutting apparatus which use a water jet ejected from a nozzle under a pressure of up to 4000 bar. Thereby, it is possible to cut different kinds of workpieces, e.g. styropor, wood, fabrics, leather, rubber, textiles and many more.
It is also known in the prior art to admix an abrasive medium to this fluid jet, e.g. quartz, glass dust, corundum etc. in order to be able to cut harder materials like metal, glass, stone and the like.
According to the prior art, it has been taught to direct the fluid jet exactly perpendicularly towards the surface of the workpiece to be cut in order to get an exact and clean cut edge. This perpendicular position had to be maintained during the entire cutting process, even if the movement of the cutting jet is controlled by a CAD-machine.
However, experience has shown that this is not true; in this manner, it is not possible to achieve a cleanly cut edge. Particularly, the edges of the cut workpieces are not exactly cut, are irregularly and do not extend exactly perpendicularly to the plain surface of the workpiece.
As can be seen from FIG. 1, for example, in cutting a plate-like workpiece 1 by means of a fluid jet-like cutting medium 2 being ejected from a nozzle 3, it is not possible to achieve an exactly cut edge surface 4 because, as a result, the angle 5 is smaller or larger than 90.degree. with reference to the surface 6 of the workpiece 1. This situation is schematically shown in FIG. 1. However, if it is required to very exactly cut a workpiece, the aforementioned disadvantages occur particularly aggravatingly. If, for example, precisely shaped parts have to be cut out of a workpiece according to a complicated shape which, thereafter, have to be inserted into correspondingly negative shaped workpieces or which have to be assembled with other precise workpieces, it is of paramount importance that the cut edges exactly run perpendicularly with regard to the workpieces.
For example, if cut-out parts, e.g. letters, are to be inserted into corresponding cut-outs of a base plate in order to manufacture inlays or high relief printings, the cut edges of the letters are allowed to be inclined inwards, but not outwards. With other words, deviations from the perpendicular direction can be tolerated in one, but only in one direction and not in the other direction. However, according to the prior art, deviations from the perpendicular direction of the cut edge surface can not be avoided because no reliable and economic method exists to provide an exactly perpendicular and clean cut edge.
The only method to avoid the aforementioned disadvantages known in the prior art was to drastically reduce the cutting speed during the cutting or separating process. A cut edge surface which is practically usable can be achieved, according to the prior art, if the theoretically possible cutting speed, depending on the material to be cut and on the cutting medium used, is not really exploited, but considerably reduced. However, such a proceeding results in a considerably loss of efficiency with the consequence that the final product becomes much more expensive. The reason is that the very expensive cutting apparatuses can not be used according to their theoretical possibilities.
The fluid jet discharged from the nozzle under high pressure is directed towards the top surface of the workpiece to be cut and the fluid jet nozzle is moved along a predetermined cutting path over the surface of the workpiece under the influence of a computerized control unit. The fluid jet cuts the workpiece and escapes from the workpiece at the rear side thereof. Since the fluid cutting jet, even after having cut the workpiece and escaping from the rear side thereof, comprises a very high residual energy, it is necessary to absorb said residual energy.
It has already been proposed in the prior art to provide a water-filled pool behind or below the workpiece to be cut in which the fluid cutting jet is caught. Thereby, the water-filled pool must have a thick and resistant wall because the fluid cutting jet impinges upon the walls of the water-filled pool. Particularly if an abrasive is admixed to the fluid cutting jet, it has not been possible to avoid that the wall of the water-filled pool is damaged or even destroyed.
In order to avoid damage or destruction of the wall, it has been proposed in the prior art to provide a bed of balls, comprising a tub filled with balls consisting of chromium steel or glass. Basically, such a design has proven to work well, but has the disadvantage that it is quite large and, consequently, heavy and expensive. Furthermore, a recycling of the abrasive is difficult to perform or even impossible.